1. Field of the Invention
This invention relates to an improved process for preparing a bulged smooth surface reverse buckling rupture disc assembly in which the bulged section of the disc has an electropolished line-of-weakness recess whereby the disc will reliably reverse and fully open at any one of a wide range of overpressure setpoints.
The present invention relates generally to a reverse acting rupture disc having a laser-defined electropolished line-of-weakness recess, and to an improved method of forming a line-of-weakness recess in a reverse acting rupture disc that assures full opening of the disc upon reversal. The line-of-weakness recess may be configured and strategically located to assure full opening of the disc along the line-of-weakness recess, while at the same time preventing fragmentation of the disc upon reversal. A rupture disc blank is first pre-bulged, followed by final bulging, and then provided with a layer of a resist material. A laser is employed to remove at least a portion of the layer of resist material corresponding to a desired line-of-weakness recess in the concave face of the bulged rupture disc. The disc is then subjected to an electropolishing operation to remove metal from the lased area of the rupture disc, thereby forming a lustrous polished line-of-weakness recess in the disc of desired configuration and of a predetermined depth that is related to material thickness. In the preferred process, electropolishing is controlled to form a line-of-weakness recess defined by spaced opposed channel portions separated by a central raised crown portion wherein the channel portions are of greater depth than said crown portion. The opposed channel portions of the line-of-weakness recess provide opening redundancy and therefore increased tearing reliability along the line of weakness without deterioration of the cycle service life.
Preferably, lasing of the disc is controlled such that a relatively thin residuum of the resist material is left on the disc at the completion of the lasing operation, thus preventing any significant oxidation of the surface of the disc by the laser beam, which would impede or interfere with subsequent electropolishing of the metal. Use of a laser to define a desired line-of-weakness recess in one face of the disc allows the manufacturer to provide any one of an essentially unlimited selection of lines of weakness configurations and disc surface markings.
This invention relates to a reverse buckling rupture disc assembly that has particular utility in sanitary pressure vessel piping applications. Pharmaceutical, biochemical and food processing equipment require that sanitary conditions be maintained at all times, which necessitates frequent cleaning of the equipment, usually with steam or other sanitization agents. These processes often are operable at relatively low level pressures in which overpressures in the equipment or piping connected thereto must be relieved at pressure levels as low as about 2 psig. It is conventional to employ reverse buckling rupture discs for a variety of applications, but it has been found difficult to provide narrow range burst pressure tolerances at low overpressures.
In order to accomplish reliable disc rupture at low differential pressures, it has now been found that a required rupture specification can be met while at the same time avoiding material collection problems on the surface of the disc by subjecting the disc material to a force which deflects a segment region of the disc from the main body thereof, and by thereafter applying a force to the disc which returns the deflected segment region to its initial position whereby the metal of the deflected and returned segment region has an altered grain structure as compared with the metal of the remainder of the central bulged section. The metal of the deflected and returned segment region exhibits higher residual stress than the disc material surrounding the initially deflected segment region resulting from initial plastic deformation of a localized segment region first in one direction, and then plastic deformation of that same localized segment region in the opposite direction.
2. Description of the Prior Art
It has long been known to provide bulged reverse acting rupture discs having a line-of-weakness recess or score line in one face of the disc bulge. The lines of weakness or score lines have generally been cross scores, or a circumferential line-of-weakness recess in the concave face of the disc where the line-of-weakness recess or score line defines the area of the disc that opens upon reversal. With out a line-of-weakness recess defining the opening through the disc upon severing of the disc along the line-of-weakness recess, a bulged disc will reverse but not necessarily fully open. In the case of a circumferentially extending line-of-weakness recess, the line of weakness normally is not a continuous line, thus presenting a hinge area that prevents fragmentation of the central area of the disc upon reversal and opening. A cross scored disc forms four petals that bend outwardly upon reversal of the disc, again preventing fragmentation of the petals. Circumferential score lines or lines of weakness are preferred in low pressure applications because of the larger opening presented upon severing of the disc along the arcuate score line, as compared with a cross scored disc.
Lines of weakness have heretofore been formed in reverse acting rupture discs by a metal scoring die, use of a laser that erodes a groove in the disc, or by chemical etching to remove metal from the disc along a desired line. All of these past reverse acting discs have presented unresolved manufacturing difficulties, or have experienced operational problems in various application uses.
Metal scoring dies work harden the metal material, thus changing the grain structure and density of the metal at the score line. The material surrounding a score line formed with a metal scoring die is work hardened during the scoring process, thus increasing the brittleness of the metal and creating stress zones. The brittleness and increased stress zones of the metal limit the service life of the rupture disc as a result of fatigue cracking and stress corrosion. Metal scoring depths required for satisfactory operation profoundly alter the original bulged dome strength making it difficult to predict the pressure ultimately required to reverse the rupture disc during the initial bulging operation of the disc prior to scoring. Consequently, it is very difficult to produce a reverse acting bulged rupture disc having a score line formed with a scoring die that will both open reliably and withstand multiple successive pressure cycles.
It has also been proposed to form a score line in a reverse acting rupture disc using a laser beam. These proposals have not proved commercially satisfactory for a number of reasons. The reflectivity of the metal makes it difficult to control the penetration of the beam into the thickness of the metal and thereby form a smooth groove of uniform depth along the length of the intended line-of-weakness recess. Furthermore, lasers significantly heat and burn the disc, oxidize the material and change the metallurgy of the metal. Discs having lines of weakness burned by a laser have been found to be unsatisfactory in use, not only from the standpoint of unreliable openings at required pressure relief values, but also having undesirable cycle life.
Chemical etching of a rupture disc having a segmented resist layer defining a line of weakness has also been suggested in the prior art, as for example shown and described in U.S. Pat. Nos. 4,122,595, 4,597,505, 4,669,626, and 4,803,136. The patentee in the '595 patent suggests screen printing of a resist material on a flat rupture disc where the screen has openings presenting a pattern of the desired line of weakness. After bulging of the disc, an acid solution is sprayed onto the disc to etch a line of weakness coincident with the area of the disc unprotected by the resist material. The metal surface of the disc material is somewhat irregular and not perfectly smooth because the individual side-by-side grains have peaks with valley structure between the grains. Therefore, when an acid etchant agent is applied to the surface of the metal, that agent does not act uniformly across the surface of the metal. Instead, the etchant is more aggressive in the valleys between the grains than in eroding the higher surface peaks of the metal grains. The etchant agent contained in the valley cavities is believed to not only more rapidly erode the metal in the valley area as compared with the surrounding peak areas of the grains, but to also do so more efficiently. The attendant result of the etching process is to exaggerate the roughness of the metal surface, with the degree of surface irregularity increasing with time of exposure of the metal to the etchant agent. The rupture discs are manufactured from materials that are inherently corrosion resistant, such as stainless steel, inconel, Hastalloy-C, and monel. As a consequence, subjection of these inherently corrosion resistant materials to an etchant acid requires that the etchant agent remain in contact with the surface of the metal for extended periods of time in order to erode away a groove that typically is as much as 70-90% of the thickness of the metal. For example, if the material is 0.004 in. thick, as much as 0.0036 in. must be eroded during the etching process.
In addition, in order to accomplish reasonably efficient erosion of these corrosion resistant materials, the etchant agent chosen must be one tailored for the particular type of metal. Thus, a different acidic agent is required for each of the various metals. The specific material used to manufacture a particular rupture disc must be selected to meet the specifications of the application. Different disc applications require use of different types of metal. Therefore, when an etching process is used to form a line of weakness in corrosion resistant disc material, the manufacturer should have available an etchant agent that is most effective in eroding that specific corrosion resistant metal.
The patentees in the '136 patent describe passing the strip of metal to be etched through a suitable etching bath, where the rate of feed of the metal strip within the bath, the acidity concentration of the bath, and the bath temperature are controlled to obtain an etched groove of preselected depth. The patentees describe the remaining material at the bottom of the groove as being a flat membrane extending along the length of the etched area.
Because the surface of an etched line of weakness has been further roughened as a result of the etching process as compared with the original surface finish of the metal material, stresses imposed on the metal defining the line of weakness during cycling of the disc are exaggerated thus reducing the cycle life of the disc. The increased surface area of the roughened line of weakness groove makes the surface area less resistant to corrosive effects which further decreases the cycle service life of the disc. This is important because after a rupture disc is mounted in place, that disc may remain in that position for many years without being operated by an overpressure condition. However, if an overpressure event occurs, the reverse acting rupture disc must function reliably, throughout the service life cycles established for that disc.
Furthermore, an etching process, if attempted on a commercial basis, cannot be economically justified for a number of reasons, including the need to have on hand a specific etchant agent for each type of metal, and the inordinate time required to obtain removal of sufficient material defining the line of weakness.
In order to obtain repeatable low overpressure opening of rupture disc assemblies designed for reverse acting applications, one commercial approach to the requirement has been to provide a reverse buckling rupture disc in which a dimple is deliberately formed in the dome of the rupture disc. The dimple in the domed area of the disc is strategically located in a position such that the domed part of the disc will fail first at the area of the dimple. The disc thus reverses and opens at what has been described the prior art as an overpressure less than a disc without a dimple.
However, a dimple in the process side convex surface of the bulged area of the disc presents a cavity that serves as a collection point for food, pharmaceuticals or the like. As a result, cleaning of the processing equipment with steam or the like is difficult and may require breakdown of the components in which the rupture disc is positioned in order to insure removal of material that may have collected in the dimple.
Exemplary of a prior art rupture disc assembly having a dimple in the convex surface of the disc is Cullinane, et al., U.S. Pat. No. 6,494,074, in which a pointed tool forced against the backed up convex surface of the bulged section of a disc forms a dimple in the disc at or near the apex of the domed shape. The shape, area and depth of the dimple is said to be selectively variable. In each instance though, the dimple in the convex surface of the bulged section of the disc presents a cavity which may collect material from the process operation that is detected from a predetermined overpressure by the disc mounted in a pipe fitting leading to the processing pressure vessel. Although Cullinane et al. suggest that the depth of the dimple may be altered, but not eliminated, the patentees did not perceive that a smooth surface disc could be provided which avoids material collection problems in a dimple in the disc by forming an indentation in the disc which is then returned to its initial smooth surface position, while at the same time meeting more stringent burst specifications. Furthermore, the present improved reverse acting rupture disc and process for fabrication of the disc allows the burst pressures to be altered by selectively controlling the amount of the pre-bulging pressure, notwithstanding the elimination of a residual dimple in the bulged portion of the disc.
FIGS. 6-9 of Graham et al., U.S. Pat. No. 6,318,576, illustrate a hygienic quick breakdown and reconnection fitting conventionally used in pharmaceutical, biochemical and food processing operations, which is adapted to receive and retain a reverse bulging rupture disc assembly. The fitting includes two couplings having flanges which are retained in adjacent interconnected relationship by a quick release clamp ring.
Reverse buckling rupture discs are preferred for differential pressure applications because a reverse buckling disc will open at a pressure near the bursting pressure of the disc without producing fatigue and Failure which often time occurs with a forward acting disc when the disc is operated near its burst pressure for long periods of time. One theory of the sequence of operation of a non-knife blade reverse buckling rupture disc is explained in Mozley, U.S. Pat. No. 4,512,171.
A commercially acceptable reverse buckling sanitary rupture disc should ideally meet current ASME BPE (Bioprocessing Equipment) and 3-Λ (milk and dairy) standards, which require the equipment to be free of surface imperfections such as crevices, gouges, obvious pits, etc.